A political theorist and historian of political thought, Ryan is a recognised authority on development of modern liberalism, and especially the work of John Stuart Mill, having contributed directly to the ‘Reversionary’ school, which led to a re-examination of Mill's work from the 1970s. His academic work also takes in broader themes in political theory, including the philosophy of social science, the nature of property, the history of political thought, and liberalism of the 19th and 20th centuries.

The Philosophy of John Stuart Mill (1970): an examination of Mill's philosophy of science, its relation to his ethical thought, and a defence of the view that Mill's work is largely coherent, concentrating on Mill's A System of Logic.